It's only usefull if you already have a need for 2 lans, otherwise, it's just useless, Personally, I have a Linksys firewall/router hooked up to a hub and five machines running threw it. It seems extreemly efficient for me.
One of my friends of the other hand has a duel lan going w/ 2 network devices on every machine and one hell of a mess w/ cabling, One is suposidly for internet, and the other is for his LAN... It looks verry uneffecient to me. and it's also slower than mine.
I say that there is no good use for it. Except for maybe a cingle reason, If you are running Internet Connection Sharing w/ a cable or DSL modem, Thats the only resonable situation I can think of.